EAGLES CARRY HOW MUCH WEIGHT?
Imagine being out on the edge of a soggy field in early morning intently peering into some shrubbery for a closer peek at a small songbird. Suddenly you hear a very loud thump only a few feet away and you see a large branch weighing over ten pounds with its heavy end embedded into the soil. Curious as to its origin, you gaze upward to see an adult bald eagle veering away high in the sky. And your first thought might be…..”wow…..what if that log had hit me in the head?!”
It turns out that such an event actually happened! In the early morning light on November 4, 2015, Alex Lamine was filming Mom Berry, one of the adult bald eagles nesting on the campus of Berry College, an educational institution begun in 1902 in Rome, Georgia. The college is home to several pairs of nesting bald eagles and an army of eagle voyeurs who watch the eagles’ nesting activities on a web cam. The first eagle pair showed up on the main campus in the spring of 2012, nesting in the top of a tall pine tree right near the main entrance. Two eaglets were successfully fledged in 2013, one in 2014, and two this past summer. A second nest on a more remote campus fledged three young in 2014, but was not active this year. A bald eagle carrying a 12-pound branch?! Sounds almost impossible, doesn’t it, but it not only happened but it was captured on film as well. This observation immediately raised three questions about bald eagles and eagles in general, and set off a flurry of emails among eagle experts, including yours truly. First, did the bird actually ‘carry’ an object weighing 12 pounds? Second, how much can eagles carry in the air? And third, do bald eagles actually gnaw off limbs from trees?
Amy Ries, who writes a blog for the Raptor Resource Project raptorresource.blogspot.ca/2015/11/how-much-can-bald-eagle-carry was quite impressed with the herculean feat and to learn more about it, she passed on the observation to a number of bald eagle experts. She was inclined to think that the branch was already in a falling motion from the tree and thus, does not support an assertion that bald eagles can fly for any distance carrying a 12-pound object, especially a branch heavy at one end and light at the other, in just one foot.
James Grier, a retired professor at North Dakota State University in Fargo, was the first eagle expert to respond. Growing up in the world of raptor research with Jim throughout all of my life, I am well aware of his decades of climbing to bald eagle nests in the Lake-of-the-Woods region of Ontario to band eaglets in order to learn more about their movements and fidelity to nesting sites. He said that unlike ospreys which carry fish with both feet while also orienting it with the air flow to reduce drag, bald eagles usually just grab either prey or nest materials with one or both feet and carry it dangling and swinging, and yes, sometimes dropping it. Flight conditions are also important, the best ones being high air pressure with a steady wind, and equally critical, lots of room for a good take-off and an ability to stay airborne. Even under such conditions, Jim said that it can still be a lot of work and effort for the eagles to carry large items. He added that sometimes if eagles can get a large item into the air but not all the way back to the nest, they will stop somewhere along the way such as on higher ground, a low tree branch, or an open tree, to get rid of dead weight such as the entrails, further disassemble it, and/or even eat some of it.
“I remember being at blinds and hearing the heavy, labored wing-beats from eagles carrying large items into the nest. I could sometimes hear the flapping from a long distance out where it almost sounded like someone beating on the side of a boat it was so loud!” Jim explained, “One of the more interesting items I remember, it wasn't a big item but a duck that was still alive when the eagle brought it into the nest. The eagle had a hold of the duck by the back and was carrying it in one foot. The duck was looking around and its feet were paddling the air like mad when the eagle landed on the nest with it!”
On the weight-carrying question, Chuck Sindelar, also a long-time bald eagle expert in Wisconsin, was the next to weigh in (sorry… couldn’t help myself!). He believes that an eagle can seldom fly with any more than half of its body weight.
Jon Gerrard concurs with this feeling. He studied bald eagles in Saskatchewan with Gary Bortolotti (R.I.P.) for many years and he quotes a story from their wonderful co-authored book entitled "The Bald Eagle: Haunts and Habits of a Wilderness Monarch”. A female of a pair of bald eagles nesting on the Gulf Coast of Louisiana in the 1890s caught and carried snow geese weighing from 4.5 to 6 pounds for up to a mile and a half to their nest. But here is the key point --- the eagle was actually flying downhill! This means that the goose was caught high in the air and the eagle basically glided downward to its nest with its prey. And this was not a one-time occurrence --- more than 35 snow goose heads were found in that particular nest at one time. Since the female weighed between 8 to 11 pounds, this suggests a weight-carrying capacity of half its body weight, but for “downhill” flights only.
With all due respect to all of the aforementioned bald eagle experts, I honestly know of no one who has accumulated as many hours of watching these magnificent birds as David Hancock, the founder of the Hancock Wildlife Foundation based in Surrey, British Columbia. He basically lives and breathes ‘bald eagles’! From his late teenage days to today, David has been an avid student of these birds and he is famous for helping to pioneer the web cameras on many of their nests much to the delight of millions of eagle enthusiasts all over the world. Surely he would have some comment on this observation.
And so he did. A number of years ago, he and some assistants were three miles offshore from the Queen Charlotte Islands. They watched a male bald eagle swoop down, catch a large red snapper, and then carry it in its talons at a speed of 25 to 30 miles per hour toward an island. After about three-quarters of a mile, the eagle dropped the fish but then immediately flew down and grabbed it again. Two hundred yards later and about a half-mile from shore, it repeated this scenario, once again relinquishing the fish to the water’s surface. Not to give up on its prize though, this stubborn bird next landed on the fish and used its wings to row it to shore! All bald eagle experts will tell you that these large birds are quite good at swimming with their wings.
There’s more to this story though. Wanting to know more about the fish’s weight, David flushed the eagle off the snapper and weighed it in at one and a half pounds. He also added that the fish “tasted marvelous”!
The whole incident drove David to undertake some weight-carrying tests with some captive bald eagles. He found that for 100 yards, males could carry objects weighing two pounds, and females about three pounds. Upon hearing about this latest “branch” incident, he too felt that the bird was likely carrying it “downhill” or the branch was in a falling motion from the tree, as Amy postulated.
On a related note, I contacted Sergej Postpalsky, a raptor expert in Michigan, and I asked him what was the largest prey he had seen carried by ospreys in his 40 years of studying this species in the Great Lakes. About two pounds, he replied, and on more than once occasion. Not bad for a bird that weighs less than half of a female bald eagle!
The other aspect of the original observation focused on the ‘gnawing” behavior whereupon the eagle apparently was seen chewing on the limb to remove it from the tree. Jim Grier confessed to knowing that bald eagles do engage in that activity, but knew little else about it.
Chuck Sindelar has seen both bald and golden eagles break sticks off standing trees by hitting them with their feet with enough force to snap them off, but did not mention any observations of them actually gnawing on them to facilitate breaking them from the tree. Jon Gerrard has often seen bald eagles at Besnard Lake, Manitoba breaking off limbs in this manner, but none as big as the one collected by the Berry College eagle. He added that they are usually dead limbs. Jon also wondered whether the eagle in question actually did some gnawing at the thick end of the branch before breaking it off because this would not fit with the fact that the eagle was clutching the thin or outer end of the limb before dropping it. He suggested that perhaps the bird gnawed the limb part way through at the thick end, and then flew to grab the thin end and then using its momentum, broke it off at the thick end. Years ago, I watched a video of ospreys in Scotland wherein the birds would dive at a tree with some speed and use their feet to snap off dead branches from trees for nesting material, but there was never any prior gnawing involved.
All in all, it was a very interesting anecdote which sparked some very healthy debate among several eagle experts. As Jim Grier points out, “With today's technologies including the eagle nest cams, more eagles around, and a lot more people watching and taking/recording pics and videos, I think we're going to get more anecdotes like this, insights into the eagles' lives that we've never seen before, and learn a lot more than we did in the past.”
I could not agree more.